How to Burn Fat Instead of Sugar and Never Bonk Again

You know what it feels like to run a 5K, a half marathon, or even a marathon. I want you to imagine that feeling now.

Now think about running three marathons, back to back to back. We’re in the realm of pretty serious ultrarunners now—this is something few people will ever do.

Now imagine doing that the next day. And the next day. And every day for the next nine days after that, totaling 1000 miles in just less than 12 days. We’re talking stupid mileage now.

That’s exactly what Stu Mittleman did. He did it by burning a better fuel than anyone else.

Why Sugar Will Only Take You So Far

Most of us run on sugar. We consume tons of it throughout the day. And since we now live a go-go-go society, we’re in a constant state of stress that tells the body it needs to burn sugar to help keep us going.

But think about this. According to Stu Mittleman, you have about 160,000 calories’ worth of energy in your body at any given time. Of that, only 4500 to 5500 calories are in the form of sugar, and a lot of that is reserved for your brain and nervous system.

That doesn’t leave much for distance running. The way most of us run, those sugar reserves are quickly depleted, at which point the options are (a) stop running; or (b) refuel with more sugar. If you don’t do one of the two, your body physically shuts down, as a way of hanging onto what little sugar it has left for brain function. And that’s what we call a bonk.

Since (a) isn’t an option for crazy NMA readers like you, you’re left with refueling as your only choice. The problem with that, though, is that when you eat more sugar, you encourage your body to burn even more of it. Soon, you’ve got to fill up again, and eventually you’ve sucked down so many gels that your stomach and GI system plot a coup to overthrow whoever is in charge, which happens to be you.

There’s an alternative to this vicious sugar cycle. It’s called burning fat, and—surprise—you’ve got plenty of it to burn (sorry, you do).

Why Burning Fat Is Phat

Remember those 160,000 calories you’re holding onto? Well, something like 85% of that is fat.

I suppose this could be bad news if your goal is zero percent body fat and the resulting death. But it’s great news if you want to run far: If you can find a way to tap into fat as your primary fuel source, then the distance you can run will be limited by muscle failure or injury long before your fuel source runs dry.

That’s how Stu Mittleman ran 1000 miles in 12 days, and it’s how he ran across the country in about 50 days. And lucky for us, Stu and others like him aren’t shy about sharing how they do it.

How You Can Train Yourself to Burn Fat for Fuel

It’s possible to change the way you run and eat so that your body learns to run on fat from the very start of your run, rather than waiting until sugar supplies are depleted, shifting to fat only as a last resort. Pretty exciting stuff, huh?

Hold it right there. Before you swear off sugar and start packing your old energy gels in your kids’ lunches, remember: This is a gradual process. If you currently take in a lot of sugar before and during your runs and you suddenly stop supplying it to your body, you’ll bonk, and it’ll be dangerous. Introduce these concepts slowly and gradually, and always carry a few gels with you for emergencies.

With that out of the way, let’s get into the nuts and bolts of what you can do to start burning fat.

First, note that we’re only talking about the long, slow run. Your body starts sugar-burning as your exercise intensity crosses the lactate threshold. (A good indicator of when this happens is when it becomes difficult to carry on a conversation, or when your mouth drops open to start taking in air while you run.) You can gradually increase the level of intensity at which you cross the threshold, so that you can eventually run faster while staying in a fat-burning state. For speedwork and hill workouts, your body will still rely on sugar, and that’s fine, since they’re short, and sugar is great for hard, short runs.

Extend your warmup period. If you’re standing still and you suddenly bolt off running, your physiology changes. Your body senses something is up (perhaps you’re being chased by bears and zombies?) and starts burning the sugar fires, since sugar is great for short bursts of energy. But that’s exactly what you don’t want to happen on your long run.

So warm up extremely slowly. Walk for the first few minutes. Then start running so slow that you have to hide your face when you pass people you know. Relax everything and enjoy it. Over the course of 10 or 15 minutes, build up to your long-run speed. Speaking of which…

Run slow! You want to stay below your lactate threshold for as long as possible, so your body can get used to burning fat for fuel. So go really slow. If you use a heart rate monitor, stay at 60 to 70 percent of your max. Make sure you easily carry on a conversation while you run. Your goal is to do this enough that your threshold increases, i.e., you can run faster yet still stay in this aerobic, fat-burning zone.

Practice running in a carbohydrate-depleted state. Running coach Greg McMillan has a great article about depriving your body of carbs so that it learns to burn fat. This means restricting sugar intake both before and during your long runs.

Stu Mittleman personally told me that he would never even eat a banana while he was running, and instead carries raw almonds and vegetable purees with him. He recommends fatty, alkalizing foods and a little protein. So besides nuts, you might also try nut butters on vegetables, or perhaps avocados and even oils if you can stand taking them straight.

Since you’ll be drinking water instead of sugary sports drink, you’ll need to replace electrolytes. For that, you can get salt pills or use something like Nuun in your water.

Again, be careful with this. I’ve found that it’s a slow process to transition to completely carb-free runs. I’m at the stage now where I’ve eliminated a lot of the sugar from my long-run routine, but I still eat some non-sugary carbohydrates. This is why I’m a big fan of pinole, and also things like whole-wheat pitas with hummus, or a wheat bagel with almond butter or peanut butter, though I’ve tried to limit gluten recently. Keep in mind that this is still very much a transition phase, as complex carbs are ultimately converted into sugar before they’re used for energy.

A Better Way to Run

You’ll probably find, as you start consuming less sugar, that running becomes more enjoyable. The sugar-burning state is a stressful one, one in which other processes in your body slow down as part of the fight-or-flight response that sugar is so well suited for.

You’ll find that your mind calms. You become more relaxed and more creative while you run, and the whole experience is more spiritual and more enjoyable.

What I didn’t tell you was where Stu ran his 1000 miles. He did it on a track. Yes, a track.

Even if you were physically able to run 75 miles a day 12 days, would your mind be able to handle that? You can tell me you don’t care about running 1000 miles, and that even 50 sounds like the worst use of a Saturday known to man. But don’t even try to tell me you couldn’t use the mental strength that Stu’s feat. And that’s what kicking the sugar habit, even in your everyday life, can do for you.

This post is part of a series of posts designed to teach you how to run long and strong. Go check out the rest!

Ever wished there was just a roadmap to guide you to the finish of your first marathon, starting from where you are now?
The No Meat Athlete Marathon Roadmap covers everything you need to know to train for and run your first marathon on a plant-based diet, including:

Great post! I am a big believer in this way of training. I started running in the good old days, before there were Garmins, MP3 players, fancy shoe technology and GELS! I have always run on an empty stomach with no fuel and it’s never been a problem. You can totally train yourself to do this and you will be very happy with the results!

Great post, Matt. I’ve been following you on your website since challenging myself to go vegetarian for one whole month a few months ago. Though I’ve since decided to re-introduce fish and poultry into my diet, 40% of my meals continue to be vegetarian. As a runner addicted to reading anything and everything related to running/health/nutrition/fitness, your posts are, by far, my favorite to read – a good mix of practical advice, motivation, and humor. I started training for my first marathon (LA Marathon) this week and your website will certainly be coming in handy in the weeks ahead. Keep up the great work!

PA, sorry about that. It’s set to only display one time, but if your browser is set to clear your cookies every time you close it (like I think it was by default on my netbook), then it’ll display every time. So just set it not to clear cookies right away and see if that works.

Another option is to subscribe to the posts by email or in Google reader. Then you don’t have to actually visit the site.

Can you tell me how it displays on your netbook that you can’t get past it? On mine, I just click the X to close it and it closes.

Hi Matt, very informative post again, thanks! When in marathon training mode, I got to a point where I could do 18-20 milers on minimal sugar fuel, so I’m very curious what adding some fat and protein could do. Right now I’m taking it easy and only running single digits at an easy pace though, so I figured it would be a good time to tweak my form and nutrition a bit. I have one (pretty gross) question: whenever I try to do this sugar depletion thing before and during a run, my sweat ends up smelling of ammonia (?) which I believe is caused by the byproduct of protein break down..so essentially, I’m burning muscle for fuel. Along with sugar and hopefully fat, of course. Does that mean I’m doing something wrong or is that just all part of the learning curve of my body that needs to tap into a different source of energy? Any input is very greatly appreciated, thanks for a great post again!!

Zuzanka, I wish I could help you, but I honestly have no idea what the answer is to this question. If it’s true though that your body is burning muscle instead of fat or sugar, I think you might want to try something else. I guess it’s possible that’s part of the transition from sugar to fat, but I’ve never heard about that in any of the sources I’ve used to learn about this.

I have also experienced this phenomena on occasion while cycling. Seems to occur when I’m not eating during longer workouts. I hadn’t been intentionally trying to burn fats or anything. No insight, just thought I’d share my experience.

It means that you are metabolising protein, its pretty common. Its likely that the majority will be from dietary protein. I would recommend keeping protein in your diet high before running to minimise the chances of catabolism, and after running to reverse any catabolic muscle loss.

“A high-protein, low-carbohydrate diet is generally what leads to sweat stinking of ammonia. Basically, it works like this. When someone with this sort of diet begins to exercise, his or her body is quickly forced to turn to proteins for the necessary energy. To do this, amino acids are broken down into various components, parts of which are converted into glucose. Other elements that come out of the process are waste products, and if the body can’t handle everything being sent its way, the leftovers are excreted out through the skin. Ammonia is one form that ready-to-go waste can take.”–http://health.howstuffworks.com/skin-care/information/nutrition/ammonia-sweat-smell.htm

So, is the ammonia smell a bad thing or not? If I train my body slowly to burn proteins instead of carbs, will I avoid building up the ammonia? I mean, I haven’t even smelled that, but so far I’ve also run on gels and stuff. I’m curious about the fat burning for fuel, though and would like to try it, but I don’t want to do anything that could be bad for my body.

Matt awesome article it really got me thinking about a lot of different things.

I do have a few questions about the 160k worth of calories that are stored in our bodies, where does the 160k come from? For example I am around 145 pounds and have a body fat percentage somewhere around 7-9% doing the math assuming 1 pound is 452 grams and 1 gram of fat is 9 calories I come out to around 52852.5 calories. I know we can burn some muscle and on Stu’s website he says that stored fat accounts for 85% so if you take that into account I come out to 62179.41 calories. Mind you that’s still a TON of calories and given at my weight I would burn somewhere around 2900 calories for a marathon; so I could still run very far. But anyways where does the extra 100k go? Is he referring to typical american who might have a higher body fat % and weight more than I do? I did some calcs and someone who weights 210 pounds and is 20% body fat that would around 170k calories stored in them, so I am thinking he might be referring to someone in this weight range.

Hey Daniel, great question. I’ve never tried to calculate it, only used the figure I’ve heard Stu cite and seen in his book. Your calculations seem correct to me; it seems like the “average” person explanation might be it. His book is aimed not at hardcore runners and athletes, but more average people, so that’s likely who he’s talking about.

Siobhan, check out the link to Greg McMillan’s article. On his site he talks about some of what he does post-workout. As far as I can recall though, I don’t think what you eat matters nearly as much as before or during, in terms of teaching your body to burn fat for fuel.

As for other workouts, I eat plenty of sugar before and during them. This is only for long runs, where you’re staying in an aerobic zone, below your lactate threshold.

Hi Matt!
I am in love with this blog. A few months ago I started experimenting with carb/sugar free training and I must say that my running is so much more enjoyable. Everything in this post is what I have found with myself. No more crazy hunger spikes and a more even blood sugar and energy level. One question for you though, What to eat DURING run that is not a sugar/carby gel? That is one thing I have yet to find something easy and quick like a gel…nuts just won’t do it during a run! I will share your blog on my blog! 🙂

I have been training this way for over a year now. I do all runs this way; tempos, sprints, and long runs. When race day comes I consume quick carbs and complex carbs and I haul ass. I believe my body burns fat and carbs as fuel on race day. Try it out!

Hi Matt – loving the info. Due to knee injuries I am applying your welcome advice to swimming – and finding my ability to swim longer distances is consistently increasing as the weeks go by. I really need to get an NMA swim cap or something.

I have to mention though, that I found the title to this article rather provacative and a little concerning especially the part where you state ‘Never Bonk Again’. This is because I am based in England and to ‘bonk’ means to *ahem* have sex.

I have to say I am mightily relieved that it appears to have another meaning… 🙂

I usually don’t eat anything before a long run, by long I mean 10-14 miles. I will typically have a gu with caffien about 45-60 minutes in which makes me feel better but that is all I will have. Pre-run if I have anything at all its usually a potato, banana, or a protien shake but thats based on what type of nutrition I had the night before, typically I just go run and if Its less than 8 miles I for sure wont eat anything at or have anything during the run other than water and electrolytes.

So you’re basically putting your body into a state of ketosis? Every time you run? Not sure that is really the best health wise… unless you have a lot of fat already…Or you are taking in a very high fat diet. Even so I don’t think I really want to put my body into a state where it thinks it’s starving.

I think the ammonia smell some people have been experiencing is from ketosis, as part of the process of ketosis turns the ketone bodies into acetone, which is excreted in the breath, sweat and urine.

Humans are actually supposed to eat carbohydrates, yes we can get by without them but if 6 billion plus humans on the planet teaches us anything its that we are an adaptable bunch – hence you have people who live on nothing but fruit and people who live on nothing but raw meat and fat. I personally will not be giving up my wonderful complex carbs – they are delightful delicious things. I will take happiness over the ability to run 1000 miles in a couple of days.

Great article, explained in perfect detail to people get the fat burning mode correct. We would love to share this article with our patients as well, but the title ‘How to Burn Fat Instead of Sugar and Never Bonk Again’ who in their right mind would never want to (blush) bonk ;-(. Might be a brit thing and all of you might be understanding something else.

well now you know ;-), I asked Dr. Ron if we could link some questions on our public question board to this article, and it was best not to. Many even in the US relate the word as the brits do as well. It is a great article though!

I enjoyed this read and also the read about your modifying the run less run faster training program to help qualify for Boston. Does this sorta go against the traiing that you modified. My dilema is this: I want to get faster. My first marathon was 4:26:31 in Hartford this past year 2011. As I want to get faster I also want to get the body trained to lose / burn more fat than sugars. I think I did this good last time because my marathon had hardly any degredation even after the 20 mile marker. I am currently training for my first 50k in April 29th. Im a 230 and would like to actually cut some more weight to help with race times.

So what’s you or anyone elses recommendation out there? Should I start getting faster on my long runs or keep them slow to true LSD form or mix it up?

Thanks for any info / help and I appreciate this webpage Tons of good info out there! 🙂

i’m a wogger – i alternate 2-3 minutes walking with an equal number of minutes of running as a way to go easy on my knees. is wogging compatable with this kind of fat-for-fuel method? i gots lots of energy reserves just waiting to be used…

Honestly, I’ve trained using the mcmillian method of not fueling during long runs but you must go extremely slow which really doesn’t calculate to becoming the fast and winning if that is your ultimate goal. If you are doing at all at a slightly higher heart-rate than yes if your body doesn’t have sugar to use it will then go to the next easiest source which is muscle. You will burn some fat also but that doesn’t mean you have to go to a low carb diet to do so. I learned the hard way and lost 10 pounds of lean muscle in a year. I now follow a high carb low fat vegan diet and am training and about to finish my first full ironman. I’ve been vegan for 5 years now and have finished 9 marathon, and numerous triathlons. If you want to train your body to burn fat as fuel do workout sessions first thing in the morning without any thing to eat but make sure your well hydrated. I’m sorry but I still don’t agree with this whole no carb/sugar approach. you just don’t ever see people out doing long distance running or triathlon events without sugar.

I highly encourage people to check out 80/10/10 by Dr. Douglas Graham which is a book about raw vegan diet and athletes and the website 30 bananas a day. You don’t have to go fully raw which I know is a very big step after choosing to be vegan but a high carb lifestyle can still be done right with high quality cooked food.

My first comment on your great site!
I have been running Ultras for sometime now and the above advise is so important and true.
I am currently training for a 24hr race here in New Zealand, and the first time as a “No Meat Athlete”!!
I will let you guys know how my training is going without the M word!

Hi there. As a runner, I used to experiment with runs and found I could run 23km even on low calories and losing weight reasonably easy. It was still a conditioning issue for me that forced slower speeds- not so much my cardiovascular fitness etc. I found the carb gels quite sickly but ofcourse they worked and took 1 every 40mins. I was 180-190lbs though, so prob could have had them every 35mins. Its a matter of preference, on halfa’s 21km runs or less I dont think anything needs to be taken, the body can learn to burn fat. I run with a 58yr old fit runner ,who apart from a high carb pasta dinner (night before) and a decent breakfast- 2.5hrs before the run- did a 95min 21km- and he was 58yrs young :). Cheers

The info in this article really makes sense, and as a new runner I am hoping to shed a few pounds. My question is, how can I go about incorporating this method of training with the “barefoot running” style of running? I do run in vibrams and as foolish as they look they really do force you to change the way your footfalls hit, causing faster turnover. How can you keep to this slower pace while not compromising footstrike?

Interesting point of view. I can understand doing some runs on low carbohydrate diet but… Depleting your muscles and liver long term of glycogen can’t be good. I think for fat burning and weight loss it works but you will positively lose muscle in the process as stated before w/ketosis. I prefer a more of a raw food approach and ingesting fruits/veggies don’t bother my stomach like gu’s. I also love watermelon during long races. Sprinkle a little salt on it and its a beautiful thing:)

Have any of you read the books by Phinney and Volek on ketogenic fat burning? I am in deep ketosis and have done several ultra distance audax rides (600km +) without on ride food of any sort . I have a 1200km ride in90hrs planned for next week. Conversion, far from being a gradual process, is very much cold turkey.

Great post- since I’ve started running ultras I’ve found that sugars just don’t seem to sustain me for very long but I’ve not been sure about how to fix this. I’ll definitely try the other types of foods- are there any suggestions for food that won’t fall apart when running? Thanks

Good read, it made me think a bit!
Since I started doing trail running more seriously, two years ago, I always felt that I prefer slower and longer runs. For this reason I also decided to do fewer races and settle on just 2 or three trail marathon races a year. I feel that the short races stress my body too much.
When I find my cruising running rhythm I feel, and indeed I can, run for 2-3 hours without exhausting me and enjoying every bit of it. I call it a “pseudo static” state.
I was not aware that I might be burning fat. It seems that without realizing it, as I got a better runner I started to burn more fat. In time I felt less and less the need to eat sugary stuff during my runs. A good portion of whole grain oats with yogurt, sweetened with a spoonful of honey a coupe of hours before the run keeps me going easily for 30 kilometers.

Very intr article!
How far in advance of the should one not eat tho? I usually can’t eat anything <2hr prior.. And I supplemented with this article http://www.marathonguide.com/training/articles/mandbfuelonfat.cfm and here I think it reads 2hrs.
But I did try to run the other day having hardly eaten any earlier that day and I could only go for 45min (60-70% MHR) cause my tummy hurt so bad. Lol.

This is such an important thing to get into your training.
I was just wondering, the way your Marathon Roadmap is structured, do you point out how to train (%MHR etc) for most effective fat fueling?
I'm rambling, forgot what my point was… Would anyway love more information on the subject 🙂
Kind regards from Norway

What? You eat when you run? Why is everyone doing this on a training run?

When I’m training, I never eat when I run and I don’t usually eat the 3 hours before a run. In fact, I can go 16 hours without food, get up in the morning and run 12 miles, No joke. Your body has more than enough fat on it to be able to do this. So, I don’t know why everyone is eating so much “fuel”…sugar.

Hi,
The wolds most successful runners like the Kenyans and Taramuharians all run 80% grains, corn etc.
In the west we have many Marathon runner death as our diet is so unnatural. Again the Kenyans, Taramuhariens etc do nut dye of a sick cardiovascular system. Carbs are our natural fuel/energy.
Kind regards
from Santa Fe, NM

Hi Matt interesting read
don’t get me wrong as a 98kg or 216 of your US lbs. runner my knees love this idea. Though 15 years ago I achieved this state when by body decided it didn’t want to make insulin anymore. Unintentionally (I wasn’t a big guy back then ) I lost 56 lbs. in three weeks, the sugar/carbs I consumed didn’t reach my cells and muscle and fat fell off me. Two days before hospital admission I couldn’t finish a 5km jog around my home town, I bonked ;-). Yes you can surly say I didn’t have a gradual transition and now 10 000’s injections later I’d gladly love to burn off some stored calories that I just can’t seem to shift. Would I try this, there are ways using incorrect insulin doses? No I wouldn’t. My nutrition isn’t perfect but I think you are trying to give yourself Keytosis and take it from me it’s not a pleasant thing. So five years on are you still following this plan?

Why can you not use this way of running for shorter runs like 10km or 5km? If your diet is always the same then you aren’t loading the body with sugar and so to run a 5km or 10km your body is still being fuelled the same way as if you run longer distances.

Great post!! Thank you for sharing all of that info! I have started training for The Rut 50k which is a tough race up Lone Peak in Big Sky, MT. I have been curious about following a diet like this during my training, but wasn’t quite sure how to go about making this switch. Your post was extremely helpful! Thanks again for sharing 🙂

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[…] rest as needed. During the race I worked on keeping my heart rate down and trying to focus on burning fat for fuel, rather than hoping from one aid station to another, eating cookies and junk hoping my sugar high […]

[…] rest as needed. During the race I worked on keeping my heart rate down and trying to focus on burning fat for fuel, rather than hoping from one aid station to another, eating cookies and junk hoping my sugar high […]

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